1. Field of the Invention
This invention relates to a fuel injection system for an internal combustion engine, and more particularly to a fuel injection system for an internal combustion engine in which so-called stratified charge is accomplished at least during light load operation of the engine.
2. Description of the Prior Art
There has been known a stratified charge technique in which a fuel injection valve is provided in an intake passage and fuel is injected in the latter half of the intake stroke during light load operation of the engine so that rich air-fuel mixture surrounds the spark plug and lean air-fuel mixture surrounds the rich mixture, and so that the air-fuel mixture is given a swirling motion as it enters the combustion chamber, thereby suppressing diffusion of the stratified fuel in the compression stroke. See Japanese Unexamined Patent Publication Nos. 56(1981)-148636 and 58(1983)-85319, for example.
In the stratified charge, the mixture near the spark plug must be rich enough to be ignited by the spark plug, but the mixture remote from the spark plug may be very lean. Thus the stratified charge is advantageous in that the overall airfuel ratio may be lean and accordingly the fuel economy can be improved and exhaust emissions can be reduced. Further, in the stratified charge, the mixture in the end gas zone is lean and hard to ignite and accordingly spark knock can be prevented.
On the other hand, there has been proposed a novel internal combustion engine in which a recess is formed in the cylinder head at a portion circumscribing the spark plug and one of the intake port and the exhaust port, and a combustion chamber is formed by the recess and a squish zone formed between the inner surface of the cylinder head and the top surface of the piston in the top dead center, the recess forming a main combustion chamber. In this engine, the burning rate can be increased by virtue of the substantially spherical and compact main combustion chamber and thereby stability in combustion can be improved.
When said stratified charge is applied to the engine, there arises a problem that a squish flow is produced in the combustion chamber as the piston moves upward and fuel accumulated around the spark plug is scattered by the squish flow to break up the stratification.